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Qualitative decreases in maternal brain size have been observed late in pregnancy. Three-dimensional T1-weighted MR volume images were obtained in nine healthy participants before and after delivery. Both the healthy and preeclamptic groups had a reduction in brain size during pregnancy that was maximal at term and that reversed by 6 months after delivery.

The brain decreases in size during pregnancy and increases in size after delivery. The changes follow a consistent time course in each woman. The mechanism and physiologic importance of these findings are speculative at the present time.

SOURCE:

https://pubmed.ncbi.nlm.nih.gov/11827871/#:~:text=Conclusion%3A%20The%20brain%20decreases%20in,speculative%20at%20the%20present%20time.